In case replacement of the material to be injected with one of a different kind is made while the injection molding machine is in operation, the existing material remaining in a screw cylinder in the molding machine must be purged completely. It is publicly known that this purging is made automatically and a variety of means for effecting such purging have been proposed and carried out effectively. In particular, as for the purging means, it has been recognized effective to apply a high pressure to a resin (material for injection molding) remaining in the screw cylinder, and there are many technical means for automatic purging based on this principle.
For example, the techniques described in publications of Japanese Laid-Open Patent Applications Nos. SHO-57-57926, SHO-58-158228, SHO-59-87136, SHO-59-87137 and SHO-60-175617 require application of additional pressure into a screw cylinder.
However, the prior art examples disclosed in these publications have several points desired to be improved from the viewpoint of mechanism and operation thereof.
Stating in brief, the techniques described in the Japanese Laid-Open Patent Applications Nos. SHO-57-57926, SHO-58-158228 and SHO-59-87136 require troublesome operation of attaching a resin replacing block or a purging container when an automatic purging is made. Further, in the techniques described in Japanese Laid-open Patent Application No. SHO-59-87137, there is a problem in that an arbitrary material among different kinds of materials cannot be injected into the mold because molds having different draft angles and adapted to set different thicknesses of final products need to be used depending on the material to be subjected to injection molding. Further, the techniques described in the Japanese Laid-Open Patent Application No. SHO-60-175617 require provision of a separate material supply means for increasing the material supply pressure with the result that the apparatus per se becomes complicated and increases in the equipment cost and in the number of manufacturing processes are unavoidable.